A variety of implantable medical devices for delivering a therapy and/or monitoring a physiological condition have been clinically implanted or proposed for clinical implantation in patients. Some implantable medical devices deliver electrical stimulation to, and/or monitor conditions associated with, the heart, muscle, nerve, brain, stomach or other organs or tissue. Some implantable medical devices employ one or more elongated electrical leads carrying stimulation electrodes, sense electrodes, and/or other sensors. Implantable medical leads may be configured to allow electrodes or other sensors to be positioned at desired locations for delivery of stimulation or sensing. For example, electrodes or sensors may be carried at a distal portion of a lead, which may be implanted at the desired location. A proximal portion of the lead may be coupled to an implantable medical device housing, which may contain circuitry such as signal generation and/or sensing circuitry.
Some implantable medical devices, such as cardiac pacemakers or cardioverter-defibrillators, provide therapeutic electrical stimulation to the heart via electrodes carried by one or more implantable leads. The electrical stimulation may include signals such as pulses or shocks for pacing, cardioversion, or defibrillation. In some cases, an implantable medical device senses intrinsic depolarizations of the heart, and controls delivery of stimulation signals to the heart based on the sensed depolarizations. Upon detection of an abnormal rhythm, such as bradycardia, tachycardia or fibrillation, an appropriate electrical stimulation signal or signals may be delivered to restore or maintain a more normal rhythm. For example, in some cases, an implantable medical device may deliver pacing pulses to the heart of the patient upon detecting tachycardia or bradycardia, and deliver cardioversion or defibrillation shocks to the heart upon detecting tachycardia or fibrillation.
Implantable medical leads associated with an implantable medical device typically include a lead body containing one or more elongated electrical conductors that extend through the lead body from a connector assembly provided at a proximal lead end to one or more electrodes located at the distal lead end or elsewhere along the length of the lead body. The conductors connect stimulation and/or sensing circuitry within the connected implantable medical device housing to respective electrodes or sensors. Some electrodes may be used for both stimulation and sensing. Each electrical conductor is typically electrically isolated from other electrical conductors, and is encased within an outer sheath that electrically insulates the lead conductors from body tissue and fluids.
Cardiac lead bodies tend to be continuously flexed by the beating of the heart. Other stresses may be applied to the lead body during implantation or lead repositioning. Patient movement can cause the route traversed by the lead body to be constricted or otherwise altered, causing stresses on the lead body. The electrical connection between implantable medical device connector elements and the lead connector elements can be intermittently or continuously disrupted. Connection mechanisms, such as set screws, may be insufficiently tightened at the time of implantation, followed by a gradual loosening of the connection. Also, lead pins may not be completely inserted. In some cases, changes in leads or connections may result in intermittent or continuous short circuits, open circuits, or changes in lead impedance.
Short circuits, open circuits, or significant changes in impedance may be referred to, in general, as lead related conditions. In the case of cardiac leads, sensing of an intrinsic heart rhythm through a lead can be altered by lead related conditions. Structural modifications to leads, conductors, or electrodes may alter sensing integrity. Furthermore, impedance changes in the stimulation path due to lead related conditions may affect sensing and stimulation integrity for pacing, cardioversion, or defibrillation.
In many cases, an implantable medical device system monitors the impedance of the one or more conductors in an implantable medical lead to detect whether a lead related condition, e.g., a fracture of one or more of the conductors, has occurred. Typically, an implantable medical device periodically, e.g., on the order of once per day, measures impedances of a plurality of electrical paths that each include one or more of the conductors. In some examples, lead related conditions can be detected by measuring the impedance of an electrical path between two or more electrodes on one or more leads, the electrical path including one or more conductors within the one or more leads. Changes in impedance of an electrical path can be indicative of a fractured conductor, or insulation failure, as examples.
The implantable medical device or another component of an implantable medical device system compares the measured impedance to one or more thresholds. A threshold may be a predetermined threshold, e.g., based on a specification of a lead or a baseline impedance measurement taken at or shortly after implant of the lead, or may be an adaptive threshold determined based on a number of previous impedance measurements, e.g., a mean or median of the N most recent impedance measurements. In either case, the implantable medical device system may identify a lead related condition if, for example, the measured impedance exceeds a threshold, and may take some action based on the identification, e.g., provide an alert to the patient or a clinician.